(1) Field of the Invention
The present invention relates to a Raman amplifier and an optical transmission system for supplying excitation lights to an amplifying medium on an optical transmission path, and particularly, to a technique for reducing characteristic deterioration caused due to excitation lights mixedly existing within a wavelength band of signal light.
(2) Related Art
In conventional long distance optical transmission systems, there has been adopted an optical regenerating repeater for converting an optical signal into an electrical signal, and for reshaping, retiming and regenerating the electrical signal, to thereby perform an optical transmission. However, with the recent development of the practical use of optical amplifiers, there has been investigated an optical amplifying-and-repeating transmission system for utilizing an optical amplifier as a linear repeater. Namely, by replacing the optical regenerating repeater by an optical amplifying repeater, it is expected that the number of constituent parts within the repeater are drastically decreased, and the cost is drastically reduced while ensuring reliability.
As one of methods for realizing a large capacity of optical transmission system, attention has been directed to a wavelength division multiplexing (WDM) optical transmission system for multiplexing optical signals of two or more different wavelengths to transmit to one transmission path.
In a WDM optical amplifying-and-repeating transmission system combining the aforementioned optical amplifying-and-repeating transmission system with the WDM optical transmission system, it is possible to collectively amplify WDM signal lights by utilizing an optical amplifier, thereby enabling to realize a large capacity and long distance transmission with a simple configuration (economical merit).
FIG. 25 is a view showing an exemplary configuration of a typical WDM optical amplifying-and-repeating transmission system.
The system of FIG. 25 comprises: for example, an optical sender station 101; an optical receiver station 102; an optical transmission path 103 that connects between the optical sender station and the optical receiver station; and a plurality of optical repeaters 104 arranged on the optical transmission path 103 at required intervals. The optical sender station 101 includes: a plurality of optical senders (E/Os) 101A that output a plurality of optical signals of different wavelengths, respectively; a multiplexer 101B that wavelength division multiplexes the plurality of optical signals into a WDM signal light; and a post-amplifier 101C that amplifies the WDM signal light from the multiplexer 101B to a required level, to thereby output the amplified WDM signal light to the optical transmission path 103. The optical receiver station 102 includes: a pre-amplifier 102C that amplifies the WDM signal light of respective wavelength bands, that has been transmitted via the optical transmission path 103, to a required level; a demultiplexer 102B that demultiplexes an output light from the pre-amplifier 102C into a plurality of optical signals corresponding to the wavelengths thereof; and a plurality of optical receivers (O/Es) 102A that receives to process the plurality of optical signals, respectively. The optical transmission path 103 includes a plurality of repeating sections that connect between the optical sender station 101 and the optical receiver station 102. The WDM signal light sent from the optical sender station 101 is propagated through the optical transmission path 103, is optically amplified by each optical repeater 104 arranged in each repeating section, and then, is propagated through the optical transmission path 103, and is transmitted up to the optical receiver station 102, while repeating the above propagation and amplification.
For each of the optical repeaters 104 of the aforementioned WDM optical amplifying-and-repeating transmission system, there is typically adopted an erbium-doped fiber amplifier (EDFA). A gain wavelength band of such an EDFA is a 1.55 xcexcm band (C-band), while a gain wavelength band of a GS-EDFA (Gain shifted-EDFA) obtained by shifting a gain band of the EDFA to a longer wavelength side, is a 1.58 xcexcm band (L-band). Each of the EDFA and the GS-EDFA has a gain wavelength bandwidth of 30 nm or above. Thus, by utilizing a multiplexing and demultiplexing device corresponding to both of the C-band and L-band to thereby use together the two signal light wavelength bands, it is possible to realize the amplifying-and-repeating transmission of WDM signal light having a wavelength bandwidth of 60 nm or above.
Recently, it has been also tried to apply Raman amplification to the aforementioned optical transmission system. The Raman amplification has a characteristic with a gain peak at the frequency lower than the frequency of excitation light by 13.2 THz, in a case where a silica (SiO2) based optical fiber doped with germanium (Ge) for example, is adopted as an amplifying medium. Therefore, a Raman gain is caused at a longer wavelength side than a wavelength of excitation light. For example, a peak wavelength of the Raman gain is 1.55 xcexcm, which is shifted to the longer wavelength side by about 100 nm relative to an excitation light wavelength of 1.45 xcexcm. Thus, it is possible to freely set a gain wavelength band and a bandwidth of Raman amplification, by selecting a plurality of excitation light wavelengths to adjust an excitation light power. Namely, in the Raman amplification, in order to realize an amplifying function for a required signal light wavelength, it is important to be able to set an excitation light wavelength taking account of the shift frequency in the Raman gain. It is also possible to flatten a gain wavelength characteristic of Raman amplification, by using a plurality of excitation lights having different oscillation center wavelengths.
Specifically, in a Raman amplifier such as shown in Y. Emori, et al., xe2x80x9c100 nm bandwidth flat gain Raman amplifiers pumped and gain equalized by 12-wavelength-channel WDM high power laser diodesxe2x80x9d, OFC""99, PD19, 1999, excitation light powers and oscillation wavelengths thereof are adjusted to ensure about 100 nm as a gain wavelength bandwidth of Raman amplifier. As shown in one example of FIG. 26, typically, such a conventional Raman amplifier is constituted so that excitation lights from an excitation light source 202 are supplied to be propagated through an optical fiber 201 acting as an amplifying medium, in a direction opposite to the propagation direction of signal lights. In this exemplary configuration, for a multiplexer 203 that supplies excitation lights to the optical fiber 201, a wavelength multiplexer (WDM coupler) having ports for transmitting lights of different wavelengths, respectively. In the aforementioned Raman amplifier, as shown in FIG. 27, a plurality of excitation lights P1 to PK of different wavelengths and a plurality of signal lights S1 to SL of different wavelengths are arranged corresponding to the shift frequencies of Raman gains, to form such a wavelength arrangement that the wavelength band xcexP1 to xcexPK of the excitation lights and the wavelength band xcexS1 to xcexSL of the signal lights are separated into different regions.
Meanwhile, there has been proposed a hybrid amplifier combining a Raman amplifier with an EDFA, such as in xe2x80x9cConsideration of SRS loss and compensating method in 3-band WDM transmissionxe2x80x9d (Society Conference 2000, B-10-167, by Institute of Electronics, Information and Communication Engineer) by Yano et al. This article described a hybrid amplifier having a constitution as shown in FIG. 28, in which an optical circulator 204 is adopted as a multiplexer that supplies excitation lights for Raman amplification to an amplifying medium.
Moreover, it is important to further broaden a wavelength bandwidth of signal lights, in order to realize a large capacity and long distance transmission system. To realize such a broad band of signal lights, for a system adopting Raman amplification, as shown in FIG. 29, the present applicant has proposed that such an arrangement is effective, in which a part of the wavelength band xcexP1 to xcexPM of excitation lights overlaps with the wavelength band xcexS1 to xcexSN of signal lights so that excitation lights and signal lights mixedly exist within the same wavelength region (see Japanese Patent Application No. 2001-030053 (unpublished)). In such a case, it is also effective to adopt the aforementioned optical circulator having no limitation on the aforementioned wavelength band, as a multiplexer that supplies excitation lights to the amplifying medium.
However, there may be the following problems accompanying to a broad band of signal lights, in the above conventional optical transmission techniques adopting the Raman amplification. Namely, when the wavelength bandwidth of signal lights is broadened, in accordance with this, the wavelength bandwidth of excitation lights is also broadened, such that the signal lights and excitation lights mixedly exist within the same wavelength region as previously illustrated in FIG. 29. In this case, there is a possibility of occurrence of linear cross-talk and/or non-linear cross-talk due to Rayleigh scattering of excitation lights, in a constitution of backward excitation type using a multiplexer such as the optical circulator 204 shown in FIG. 28.
Specifically, when the backward excitation lights supplied to the amplifying medium via the multiplexer from the excitation light source are propagated through the amplifying medium, Rayleigh scattering lights of the same wavelengths as those of the excitation lights, respectively, are generated and propagated in the same direction as the signal lights. These Rayleigh scattering lights are attenuated by virtue of the transmission loss of the amplifying medium, when the wavelengths of the Rayleigh scattering lights are outside the wavelength band of signal lights. However, when the wavelengths of these Rayleigh scattering lights are within the wavelength band of signal lights, these Rayleigh scattering lights are propagated through the amplifying medium while maintaining the power thereof, since the transmission loss is compensated for by the Raman gain. For example, if a power of excitation lights having wavelengths mixedly existing within the wavelength band of signal lights is 20 dBm (100 mW), there may be caused a power of the lights returning to the multiplexer due to the Rayleigh scattering may be about xe2x88x9210 dBm (0.1 mW). This means that the Rayleigh scattering lights maintaining a power of certain level will pass through the multiplexer together with the signal lights.
Such Rayleigh scattering lights are to be generated in each repeating section, in a case of an optical amplifying-and-repeating transmission system that performs distributed Raman amplification by using, for example, an optical transmission path as an amplifying medium. Thus, as the transmission distance is increased, the Rayleigh scattering lights are accumulated to thereby have a larger power. For example, in the aforementioned case, a power of the Rayleigh scattering lights accumulated after repeated for 100 repeating sections each having a length of 50 km, becomes about 10 dBm (10 mW). As such, an optical S/N ratio of the signal lights of wavelengths adjacent to such Rayleigh scattering lights is regarded to be deteriorated due to the linear cross-talk.
Further, since the Rayleigh scattering lights of the excitation lights mixedly existing within the wavelength band of signal lights are accumulated to thereby increase the power thereof, there is a possibility of occurrence of four-wave mixing between the Rayleigh scattering lights and signal lights, such as shown in FIG. 30. Generally, the signal light power per channel in an output of an optical repeater is on the order of xe2x88x9210 dBm. However, the power of the accumulated Rayleigh scattering lights of the excitation lights may largely exceed the signal light power. Therefore, there may be a possibility of occurrence of the four-wave mixing which is one of non-linear optical effects. Further, in a case where a beat noise between the newly generated four-wave mixing light and the existing signal lights is within a base band of optical receiver, there is caused the deterioration of transmission characteristic considerably greater than the deterioration due to power cross-talk.
Meanwhile, for a technique for suppressing useless light in the Raman amplification, the removal of spontaneous Raman scattering lights to be generated when signal lights are Raman amplified, has been disclosed in Japanese Unexamined Patent Publication Nos. 2-153327 and 2000-151507. However, the wavelengths of spontaneous Raman scattering lights are shifted from those of excitation lights. Therefore, it is impossible to suppress the Rayleigh scattering lights of the excitation lights even if the above technique is applied, so it is hardly to solve the aforementioned problems. Further, there has been proposed a constitution to block the Rayleigh scattering lights of signal lights, in Japanese Unexamined Patent Publication No. 2001-185787. However, it is also difficult to suppress the Rayleigh scattering lights of the excitation lights, since the wavelengths of signal lights are different from those of excitation lights. Moreover, there has been disclosed a constitution in which an outputting unit of a Raman amplifier is provided with a filter that removes excitation lights, in Japanese Unexamined Patent Publication No. 60-263538. However, in this known technique, since the wavelengths of excitation lights are outside the signal light wavelength band, it is impossible to suppress even the Rayleigh scattering lights of the excitation lights mixedly existing within the signal light wavelength band, so it is hard to solve the aforementioned problems.
As another problem to be caused by excitation lights for Raman amplification, for example as shown in FIG. 31, in a case where a part of Raman amplified signal light is demultiplexed by a demultiplexer 205 so as to monitor the demultiplexed light by an output monitoring unit 206, it is considered that the aforementioned Rayleigh scattering lights of the excitation lights are mixed into the signal lights to act as noises, thereby being an error factor in monitoring the signal light output. Particularly, when the wavelengths of excitation lights and the wavelengths of signal lights are mixedly arranged, the Rayleigh scattering lights of the excitation lights are not attenuated but Raman amplified in the above manner, thereby increasing the power of Rayleigh scattering lights to be input to the output monitoring unit 206. The aforementioned problem is also true in a case where the wavelength band of excitation lights is different from the wavelength band of the signal lights. Namely, although the Rayleigh scattering lights of the excitation lights outside the wavelength band of signal lights are attenuated due to the transmission loss of the amplifying medium, such scattering lights may be an error factor when the output monitoring with higher precision needs to be performed.
Moreover, in a case where the excitation lights are modulated so as to transmit a supervisory signal, the output monitoring unit 206 is input with modulated signal lights and the Rayleigh scattering lights of the excitation lights such that the output monitoring unit 206 is forced to demodulate the supervisory signal from two modulated signals, thereby causing a factor of controlling error. Additionally, since the Rayleigh scattering lights are generated at various positions in the longitudinal direction of the optical fiber, it is also regarded that the Rayleigh scattering lights act as intensity modulating components, to thereby increase the controlling error. Specifically, assuming that the optical speed within the optical fiber is 2xc3x97108 m/s and the effective length of the optical fiber for the excitation lights is in a range of 10 km to 20 km, frequency components in a range of 10 kHz to 40 kHz are caused as the aforementioned intensity modulating components, such that these frequency components act as the noises in output monitoring, thereby causing a factor of controlling error.
Furthermore, as another problem to be caused by excitation lights for Raman amplification, for example as in shown in FIG. 32, in a case of a system structure in which the state of optical transmission path is capable to be measured by utilizing an optical time domain reflectometer (OTDR), such an inconvenience is considered that the excitation lights having been propagated through one optical transmission path 201 (or 201xe2x80x2) are propagated through the other optical transmission path 201xe2x80x2 (or 201) via optical paths 207 for the OTDR. Namely, during in-service, the backward excitation lights having passed through the optical path 207 for the OTDR become forward excitation lights on the opposite side optical transmission path to thereby change the gain wavelength characteristic of Raman amplification. Also, in performing the OTDR measurement, the excitation lights having passed through the optical path 207 become noises in measuring. Note, the aforementioned measurement utilizing the OTDR is a method for measuring a breakpoint of an optical transmission path, by inputting an optical pulse to the optical transmission path to observe a reflected light from the optical transmission path. This method utilizes such a phenomenon that, in case of no breakpoint of the optical transmission path, only Rayleigh scattering lights of the measuring light are reflected to returned, whereas in case of any breakpoint, Fresnel reflection light returns.
The aforementioned problems in the conventional techniques have been reviewed for the case of the backward excitation type Raman amplification. However, even in performing Raman amplification of forward excitation type where the excitation lights are propagated in the same direction as the signal lights, the same problems as those in the backward excitation type can be considered, by assuming a leaked light from forward excitation lights propagated through the amplifying medium, instead of Rayleigh scattering lights of backward excitation lights.
The present invention has been achieved in view of the aforementioned problems, and it is therefore an object of the present invention to provide a Raman amplifier and an optical transmission system capable of reducing characteristic deterioration to be caused by excitation lights mixedly existing within wavelength band of signal lights.
To achieve the above object, the present invention provides a Raman amplifier for supplying excitation lights to an amplifying medium on an optical transmission path through which signal lights are propagated, to Raman amplify the signal lights being propagated through the amplifying medium, wherein, when a wavelength band of the excitation lights has a region overlapped with a wavelength band of the signal lights, the Raman amplifier comprises an excitation light wavelength suppressing unit capable of suppressing optical components which are included in lights propagated through the amplifying medium and are of the same wavelengths as those of the excitation lights within the region overlapped with the wavelength band of the signal lights.
According to the Raman amplifier having such a constitution, the excitation lights having the wavelength band overlapped with the wavelength band of the signal lights are supplied to the amplifying medium on the optical transmission path, so that the signal lights being propagated through the optical transmission path are Raman amplified to be sent to the excitation light wavelength suppressing unit. This excitation light wavelength suppressing unit suppresses the optical components which are included in the lights propagated through the amplifying medium and are of the same wavelengths as those of the excitation lights, thereby outputting Raman amplified signal lights substantially excluding useless lights caused by the excitation lights mixedly existing within the wavelength band of the signal lights. Thus, since the aforementioned characteristic deterioration of the signal lights can be reduced, it becomes possible to realize a broad band of signal lights.
As one aspect, the Raman amplifier may comprise: an excitation light generating unit that generates excitation lights; and a multiplexing unit that supplies, to the amplifying medium, the excitation lights from the excitation light generating unit as backward excitation lights to be propagated in a direction opposite to a propagation direction of the signal lights, wherein the excitation light wavelength suppressing unit suppresses Rayleigh scattering lights of the backward excitation lights included in the lights propagated through the amplifying medium. Further, as a specific exemplary constitution, preferably, the multiplexing unit includes: a first port connected to a signal light output end of the amplifying medium; a second port connected to an input end of the excitation light wavelength suppressing unit; and a third port connected to an output end of the excitation light generating unit; wherein the multiplexing unit has a directionality capable of transmitting the light directed from the first port toward the second port and the light from the third port toward the first port.
As another aspect, the Raman amplifier may comprise: an excitation light generating unit that generates the excitation lights; and a multiplexing unit that supplies, to the amplifying medium, the excitation lights from the excitation light generating unit as forward excitation lights to be propagated in the same direction as the propagation direction of the signal lights; wherein the excitation light wavelength suppressing unit suppresses leaked light included in the lights propagated through the amplifying medium.
The present invention further provides an optical transmission system for supplying excitation lights from a Raman amplifier to an amplifying medium on an optical transmission path through which signal lights are propagated, to transmit the signal lights while Raman amplifying the signal lights, wherein, when wavelength band of the excitation lights has a region overlapped with a wavelength band of the signal lights, the optical transmission system comprises an excitation light wavelength suppressing unit capable of suppressing optical components which are included in lights propagated through the amplifying medium and are of the same wavelengths as those of the excitation lights within the region overlapped with the wavelength band of the signal lights. The excitation light wavelength suppressing unit may be provided in the Raman amplifier, a gain compensating device, an optical receiving device and an OTDR-aimed optical path.
According to the optical transmission system having such a constitution, since the excitation light wavelength suppressing unit suppresses the optical components which are propagated together with the signal lights Raman amplified by being propagated through the optical transmission path and are of the same wavelengths as those of the excitation lights, it is avoided that useless lights caused by the excitation lights mixedly existing within the wavelength band of the signal lights are accumulated. Thus, it is possible to transmit signal lights of a broad band with excellent characteristics.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.